E-UTRAN is the air interface of 3GPP's Long Term Evolution (LTE) upgrade path for mobile networks. In E-UTRAN mobile networks, the network controls a UE to perform measurement for intra/inter-frequency or inter-RAT mobility by using broadcast or dedicated control. For example, in RCC_IDLE state, a UE shall follow the measurement parameters defined for cell reselection specified by the E-UTRAN broadcast. On the other hand, in RCC_CONNECTED state, a UE shall follow the measurement configurations specified by measurement objects via radio resource control (RRC) messages directed from the E-UTRAN.
Intra-freq measurement occurs when the current and the target cell operate on the same carrier frequency. In such a scenario, UE should be able to carry out measurements without measurement gaps. This is because UE receiver is able to measure reference signals of neighboring cells while simultaneously performing data communication with serving cell in the same frequency. On the other hand, inter-freq measurement occurs when the target cell operates on a different carrier frequency as compared to the current cell. Similarly, inter-RAT (Radio Access Technology) measurement occurs when the target cell operates on a different RAT as compared to the current cell. In such a scenario, UE may not be able to carry out measurements without measurement gaps. This is because UE receiver needs to switch to another frequency to perform measurements and then switch back to the frequency of the current cell to perform data communication.
Current LTE mobile networks are typically developed and initially deployed as homogeneous networks using a macro-centric planning process. A homogeneous cellular system is a network of macro bases stations in a planned layout and a collection of user terminals, in which all the macro base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the packet core network. LTE-Advanced (LTE-A) system improves spectrum efficiency by utilizing a diverse set of base stations deployed in a heterogeneous network topology. Using a mixture of macro, pico, femto and relay base stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband user experience. In a heterogeneous network, smarter resource coordination among base stations, better base station selection strategies and more advance techniques for efficient interference management can provide substantial gains in throughput and user experience as compared to a conventional homogeneous network.
In heterogeneous networks, small cell discovery is important to ensure efficient offload from macrocells to small cells. A small cell may include a picocell, a femtocell, or even a microcell. Because of the relative small cell coverage, inter-frequency measurement time maybe too long for small cells. For example, depending on the measurement gap pattern, inter-frequency cell identification time could be up to 7.68 s, which is unacceptable for small cell discovery. Furthermore, UE may waste power if it keeps trying to search for small cells that are in spotty deployment. Note that, measurement gap may be unnecessary for UE equipped with multiple RF receiver modules. However, for such multi-RF UE, power wasting is still a concern. Therefore, it is desirable to identify and evaluate strategies for improved small cell discovery, especially for the purpose of inter-frequency mobility. The support in 3GPP specifications for closed subscriber group (CSG) cells, which are assumed to be small, has significant drawbacks for networks where a UE may visit large number of small cells, as it relies on the UE storing significant amounts of information for each individual cell where the UE is allowed access.